A differentiation block is a cardinal attribute of nearly all human cancers and leukemias. For this reason, a major focus of clinical oncology trials has been to utilize differentiating agents, such as retinoic acid, to reverse the maturational defects characteristic of malignant cells. The most successful example of differentiation therapy has been the use of retinoic acid to treat acute pro-myelocytic leukemia, a form of leukemia associated with fusion of the retinoic acid receptor alpha gene encoding the Pml nuclear protein. In acute pro- myelocytic leukemia, the leukemic cells are blocked at the pro-myelocyte stage of differentiation but can be induced to develop into mature neutrophils by treatment with retinoic acid. The broad, long-term objectives of the proposed work are to elucidate the mechanism controlling the differentiation of blood cells, to understand how multiple genetic alterations combine to result in leukemia, and to utilize this knowledge to improve oncologic therapy. An animal model of acute pro-myelocytic leukemia was created by expression of a PMLRARalpha transgene in murine myeloid cells. The specific research proposed in this application is intended to utilize this model to fulfill three specific aims: 1) elucidate the mechanisms by which the chimeric PmlRaralpha protein impairs the differentiation of myeloid cells, 2) identify genetic events that cooperative with PmlRaralpha in leukemogenesis and define the role of these additional events, and 3) elucidate the mechanisms by which retinoic acid reverses the PmlRaralpha proteins will be determined. The ability of PmlRaralpha to alter the expression of genes that may regulate neutrophil differentiation will be assessed. The nature of the vents that cooperate with PmlRaralpha will be studied in the mouse model; cooperative events will be isolated with molecular cytogenetic and proviral tagging methods. The ability of retinoic acid to reduce PmlRaralpha protein levels and to induce transcriptional changes will also be evaluated in the mice. This proposal provides the additional mentored research career development required by the candidate to achieve his immediate career goal of becoming an independent investigator and will enable the candidate to develop a productive career as an academic hematopathologist, spending a portion of his time diagnosing hematologic disorders, devoting major efforts to research in leukemia pathogenesis, and utilizing this knowledge to improve therapy.